Question : Subscription licences

I want to supply one of my databases on a monthly subscription basis.  Essentially, if the client hasn't paid, then I will want the software to stop working (until they have).  I have come up with several ways to do this, but at the same time I have come up with several ways to hack it.

Does anyone know of any software or methodologies that I could use?

Answer : Subscription licences

Here are links to Software protection technique and discussions.  Realize that any technique is hackable.
www.searchlores.org/protec/protec.htm
http://inner-smile.com/nocrack.phtml
www.activelock.com/
www.cacert.org/
http://msdn.microsoft.com/en-us/library/aa167281%28office.11%29.aspx
www.thenelson.name/#ShiftBypassDemo, "Delete the database if shift bypass is enabled."